Penile morphology sexual selection and human evolution

It is an unfortunate circumstance that so many authors have applied hyperbole to descriptions of human penile morphology. Smith (1984) regarded the human penis as 'extraordinary relative to the other hominoids'. Baker and Bellis (like Smith) stressed that 'it is nearly twice as long and over twice as wide as that of the chimpanzee'. Jolly (1999) states that a 'peculiarity of humans is that the penis is twice the size for body weight as that of any other primate.'

n3 io

Chimpanzee

• Colobus monkey

G G.5 l.G l.5 Residual testis size

n3 io luti 2.5 ol

Chimpanzee

Human

Macaque Orangutan Gibbon Colobus monkey Gorilla

Semen coagulation rating

Figure 3.18 Correlations between the rate of evolution of the semenogelin 2 gene, and A. relative testes size, and B. semen coagulation ratings in primates, including H. sapiens. Source: Redrawn from Dorus et al. (2004).

Chimpanzee

Semen coagulation rating

Figure 3.18 Correlations between the rate of evolution of the semenogelin 2 gene, and A. relative testes size, and B. semen coagulation ratings in primates, including H. sapiens. Source: Redrawn from Dorus et al. (2004).

Miller (2000), in a book dealing with effects of sexual selection on evolution of the human brain, states that 'adult male humans have the longest, thickest and most flexible penises of any living primate'.

None of these statements, and many others of a similar nature which pervade the published literature, is accurate, with the exception of the observation that the thickness of the human penis exceeds that of the apes. It is regrettable that exaggerated accounts of human penile size and shape have been used to bolster the proposition that sexual selection has moulded the evolution of the human genitalia and the psychological mechanisms that influence copulatory behaviour. Four theories have been advanced to account for this:

1. The length of the human penis has been selected to deliver sperm as close as possible to the female's os cervix during copulation, in order to gain an advantage in sperm competition (Smith 1984).

2. The human penis acts as a 'piston' during copulation and its size and distal morphology serve to displace and remove semen deposited during previous copulations, thus producing an advantage in sperm competition (Baker and Bellis 1995; Gallup et al. 2003; Gallup and Burch, 2004; see also Rolls 2005).

3. Penile size and shape evolved to impart pleasurable stimulation to the partner during copulation, and to induce female orgasm. Female orgasm influences sperm retention and transport in relation to sperm competition (Fisher 1982, 1992; Small 1993; Baker and Bellis 1993a; 1993b; 1995; Miller 2000).

4. The human penis is displayed more prominently than those of other primates and this might be partly a result of sexual selection to enhance a visual signal of attractiveness or status (Short 1980; Diamond 1997).

In order to evaluate these hypotheses, I will firstly present data on penile measurements in human beings and in other primates. Penile morphology is then considered in relation to primate mating systems and the possible effects of copulatory and post-copulatory sexual selection. Human penile size and shape will then be discussed in comparative perspective.

Table 3.4 provides data on penile dimensions in several human populations, together with reliable measurements of penile lengths in the great apes. On average, the human penis is 15-16 cm long and 10-12 cm in circumference, during tumescence. There is considerable individual variability in penile size, however, and these measurements are not correlated with men's height or body weight. There may be ethnic differences in penile dimensions but these remain uncertain, due to a lack of reliable comparative data. Potts and Short (1999) refer to a survey (conducted by Japanese prostitutes) reporting a slightly shorter average length of erection in Japanese males (13.75 cm) as compared to westerners. Rushton and Bogaert (1987) cite Nobile (1982) who used the Kinsey data to examine possible differences in penile size between American blacks and Caucasians. Measurements from Kinsey's surveys were published in a supplementary volume by Gebhard and Johnson (1979) which includes the lengths and circumferences of penes for some thousands of subjects. These data were self-ratings made to the nearest half inch (1.25 cm). Rushton and Bogaert emphasize that the length and circumference of the flaccid and erect penis is significantly larger in black males than in Caucasians. However, none of the differences they cite exceeds 0.5 in. and thus one wonders whether the methods used by Kinsey et al. might have produced

Table 3.4 Penile dimensions in man and the great apes

Species Sample size Measurement (cm) Source

Table 3.4 Penile dimensions in man and the great apes

Species Sample size Measurement (cm) Source

Homo sapiens

USA

Erect 15.0 Circumference* 10.0 Erect 12.5

Gebhard and Johnson (1979)

Wagner and Green (1981)

USA

Erect 12.9 Circumference Erect 12.3

Wessells, Lue, and McAninch (1996)

USA

52

Lengtht 12.2

Spyropoulos et al. (2002)

Czechoslovakia

177

Length* 7.2 Range 4.5-11.0 Circumference* 9.5 Range 7.7-12.0

Farkas (1971)

Nigeria

20

Length* 8.1 Circumference* 8.8

Ajmani, Jain, and Saxena (1985)

Pan troglodytes

6

Lengtht 14.0

Dahl (1988)

11

Erect 14.4 Range 10.0-18.0

Dixson and Mundy (1994)

Pan paniscus

1

Lengtht 17.0

Dahl (1988)

Gorilla gorilla

1 1

Length* 5.0 Erect 6.5

Dixson (1998a) Short (1980)

Pongo spp.

4

Lengtht 8.75 Range 7.5-10.0

Dahl (1988)

*penis flaccid.

tpenis flaccid and stretched (which approximates to the length when erect).

*penis flaccid.

tpenis flaccid and stretched (which approximates to the length when erect).

differences due to subjective bias. In the only study I have been able to locate which was conducted in Africa, Ajmani et al. (1985) recorded that Nigerian medical students had penes averaging 8.16 cm in length and 8.83 cm in circumference when flaccid. Rushton and Bogaert cite the equivalent Kinsey measurements for American Caucasians as follows: length: 3.86 in. (9.65 cm) and circumference: 3.16 in. (7.9 cm). As matters stand, therefore, it would not be justifiable to conclude that robust ethnic differences in penile size have been demonstrated in human populations. Baker and Bellis's (1995) inclusion of a smaller range of erect penile lengths in Mongoloids (10-14 cm) is likewise not supported by a critical reading of the source cited in their book (Rushton and Bogaert 1987).

All of the great apes have longer penes than originally reported by Smith (1984) in his highly influential review on human sperm competition. Smith, citing the limited information then available, reported penile lengths of 8 cm (chimpanzee), 4 cm (orangutan), and 3 cm (gorilla). As can be seen in Table 3.4 all of these length measurements are inaccurate, by more than 100 per cent in the case of the orangutan and gorilla, and 75 per cent in the case of the chimpanzee. Human penes are not longer than those of chimpanzees, therefore, although they are markedly thicker (and hence have a greater circumference) than those of the great apes. No accurate data on penile circumferences are available for apes, or for the vast majority of primate species. The erect penis was 10-18 cm long in eleven chimpanzees measured in Gabon (Dixson and Mundy 1994) and averaged 14.4 cm, close to the 14 cm reported by Dahl (1988) for measurements of the flaccid (stretched) penes of chimps at Yerkes Primate Center in the USA. Dahl (1988) also made careful measurements of orangutan penes, finding them to be much longer than generally reported, with the exception of a long-neglected account by Hill (1939). Thus orangutans have penes ranging from 7.5 to 10 cm in length (Table 3.4). In the gorilla, the penis is approximately 6.5 cm long when erect; this measurement was made from an illustration in Short (1980) which includes a centimetre scale. I have been able to record penile length in only one gorilla; it measured 5.0 cm, in the flaccid condition, in a mature western lowland specimen (Dixson 1998a).

The human penis is thus not 'twice as long' as that of the chimpanzee, nor is it exceptionally long in relation to those of other primate species, especially when their body sizes are taken into account. Figure 3.19 presents data on penile lengths and body weights for fourteen primate species, including

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Figure 3.19 Relationship between body weight and length of the erect penis in fourteen primate species, including H. sapiens. 1. Callithrix jacchus; 2. Aotus lemurinus; 3. Otolemur garnettii; 4. Lemur catta; 5. Lophocebus aterrimus; 6. Macaca mulatta; 7. M. arctoides; 8. Papio hamadryas; 9. Mandrillus sphinx; 10. Pongo pygmaeus; 11. Gorilla gorilla; 12. Pan paniscus; 13. P. troglodytes; 14. Homo sapiens.

Semen Covered Monkey Penis
Figure 3.20 Large and morphologically complex penes occur in some of the non-human primates, so that human beings are not unique in this regard. Left: a woolly monkey (Lagothrix); Right: A spider monkey (Ateles).

Source: After Dixson (1998a) and Campbell (2007).

the ring-tailed lemur and greater galago, as well as seven species of monkeys, the great apes, and human males. The erect human penis is comparable in length to those of other primates, in relation to body size. Only its circumference is unusual when compared to the penes of other hominoids. However, even this comparison may not hold true for some primates, such as the spider monkeys (Ateles) which have large and thick penes. Primate penes are rarely depicted in the erect state in the anatomical literature, and the morphology of the erect organ often differs dramatically from its flaccid condition (examples are shown in Figure 3.20). Bowman (2008) suggests that the greater thickness of the human penis may be linked to changes which have occurred in the female pelvis and vagina during evolution: 'As the diameter of the bony pelvis increased over time to permit passage of an infant with a larger cranium, the size of the vaginal canal also became larger.' Co-evolution meanwhile selected for thickening of the penis, to facilitate 'a satisfactory fit' during coitus. As we shall see, this mechanical explanation is more credible than hypotheses concerning the effects of sexual selection via sperm competition upon human penile morphology.

Is there any evidence that sexual selection has influenced the evolution of phallic morphologies in the non-human primates? The answer to this question is 'yes' and several traits are more highly developed in those species with mating systems which are conducive to the occurrence of sperm competition. Thus, primates with multi-male/multi-female, or dispersed mating systems (and larger testes sizes) have more complex penile morphologies than monogamous and polygynous forms (Dixson 1987a; 1987b; 1995a; 1998a). The differences include not only the tendency for the penes to be longer, but also morphologically more complex distally. In some cases elongation of the penile bone (baculum) also occurs.

Figure 3.21 shows representative examples of penile morphologies in non-human primates in which females have primarily multi-partner, or single partner, mating systems. Ratings of penile complexity (length, distal complexity, size of bacu-lum, and penile spines) show some consistent differences between these mating systems, as can be seen in Figure 3.22. Using a 5-point scale, each trait was rated for species representing forty-eight primate genera. The data have been taken largely from my original report (Dixson 1987a) but with additions and alterations, because more information has become available during the intervening years. This revision also includes H. sapiens, so that it is now

Primate Penis

Figure 3.21 Examples of penile morphologies in primates which have primarily polygynous mating systems (A-H), or multi-male/multi-female mating systems (I-N). A. Trachypithecus vetulus; B. Presbytis thomasi; C. Semnopithecus entellus; D. Cercopithecus mona; E. C. petaurista; F. C. albogularis; G. C. neglectus; H. Erythrocebus patas; I. Eulemur fulvus; J. Saimiri boliviensis; K. Macaca arctoides; L. M. fascicularis; M. Papio cynocephalus; N. Pan troglodytes.

Source: From Dixson (1998a); A-H: After Hill (1958; 1966).

possible to compare human penile traits with those displayed by the non-human primates.

Primates in which the females typically show multi-partner matings (multi-male/multi-female and dispersed mating systems) have significantly longer and distally more complex penes than representatives of polygynous or monogamous genera

(Figure 3.22). They also tend to have longer bacula (when present) and larger penile spines, but these traits are more variable. Larger penile spines, for example, are more common in prosimian primates than in the monkeys and apes. Nocturnal prosimi-ans commonly exhibit non-gregarious (dispersed) mating systems, but their large penile spines may

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  • nazario
    What does monkey penis look like compared to human?
    8 months ago

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